Municipal and industrial sludge and waste and other sources of waste-products of primarily organic origin such as by-products from gardening, agriculture, forestry, timber industry, food processing industries and the like, have over the years been the subject of increasing interest as possible starting materials for the production of CO2-neutral fuels such as bioethanol or bio gas.
A number of different pre-treatment methods for biomass material by which the content of sugars and the like are made more available have been described in the literature. The most well-known are: Strong and weak acid hydrolysis; wet explosion (Steam Explosion—STEX); wet oxidation (WO); basic fiber explosion (Ammonia Fibre Explosion—AFEX); and thermal hydrolysis (Liquid Hot Water—LHW).
Typically strong and weak acid hydrolysis are characterised in that hemicellulose is hydrolysed and dissolved and the availability of cellulose is increased for a subsequent acid-based or enzymatic hydrolysis. When using these types of hydrolysis it is, after separation of the insoluble and the dissolved fractions, possible to process these fractions further among others by means of fermentation. Strong acid hydrolysis has among others been described by Lightner (U.S. Pat. No. 6,258,175), where also the possibility of re-using the applied acid after precipitation with ethanol is described. The primary purpose of the process is to dissolve cellulose and hemicellulose for subsequent use in e.g. production of ethanol by means of fermentation.
There are several problems connected with acid hydrolysis of biomass. Firstly it is necessary to divide the material to very fine particles (<1 mm), which is extremely energy demanding. Furthermore, a neutralization of the treated material is required, which is normally carried out by addition of CaCO3 (limestone). This means that the consumption of chemicals in the process is high concurrently with a considerable amount of hydrated calcium sulphate being accumulated by the neutralization process. Moreover, the treated material from the acid hydrolysis has an inhibiting effect on enzyme hydrolysis and microbial fermentation compared to material resulting from other forms of treatment (see below). Finally, pumps, reactors and the like are exposed to corrosion as a result of the acid-catalysed process.
Wet explosion (STEX) was described as far back as 1928, where Mason developed the process for manufacturing hardboards (U.S. Pat. No. 1,824,221 and U.S. Pat. No. 2,759,856). The STEX process consists of thermal hydrolysis under high pressure, whereafter the pressure is released in a so-called “flash effect”, where an explosion of each fibre takes place due to the great drop of pressure—hence the name wet explosion (or steam explosion). This method of treatment has later on been further developed for the manufacture of e.g. ethanol or paper (e.g., WO 98/27269).
In STEX normally a partial dissolution of hemicellulose (>80%) takes place, and cellulose is made available for subsequent hydrolysis. The effect of STEX resembles the effect of acid hydrolysis—however, the STEX process exposes the process equipment to far lesser wear and is not so demanding as regards the use of chemicals and accumulation of waste. However, in STEX there is still a considerable formation of substances that inhibit a possible subsequent fermentation process (Palmqvist and Hahn-Hägerdal 2000) particularly if the material previously has been liquified with acid (SO2 or H2SO4 (Martin et al. 2002)).
Wet oxidation (WO) has been developed in order to oxidize organic waste fractions (U.S. Pat. No. 2,690,425) and has later on been modified so as to obtain a solution of hemicellulose from lignocellulose-containing biomass and organic waste (see e.g., WO 00/14120). Wet oxidation comprises a thermal process with addition of an oxidizing agent like an excess pressure of oxygen. In a wet oxidation the hemicellulose is partially dissolved and part of the present lignin is oxidized whereby the availability of cellulose is increased. Normally, WO does not require an extra process step for the removal of inhibiting substances.
Basic fibre explosion (AFEX) is a process that combines steam explosion and addition of a basic catalyst. In traditional AFEX the biomass is liquified in ammonia water at moderate temperatures (˜50° C.), after which the pressure is momentary released (explosion). By this process cellulose and lignin are modified, which makes the cellulose more reactive (available), concurrently with release of the hemicellulose.
Thermal hydrolysis (LHW) is a process (typically 170° C.-230° C.) in which a high dissolution of hemicellulose takes place concurrently with a partial dissolution of lignin and an improved availability of cellulose (for enzymatic hydrolysis). Waste of sugar cane that has not previously been divided and that has been pre-treated with LHW, results in up to 90% of the theoretic ethanol yield after enzymatic hydrolysis and fermentation after addition of moderate amounts of enzyme (Van Walsum et al. 1996). U.S. Pat. No. 4,461,648 describes a method that increases the availability of cellulose- and lignocellulose-containing materials. The method comprises the addition of water steam under pressure, heat treatment and wet explosion, it is further described that a recycling of steam is not possible.
Known methods for production of CO2-neutral fuels based on such organic waste or biomass often include a pre-treatment step employing some kind of Thermal Hydrolysis Process (THP) followed by an anaerobic digestion.
The processes are often based on a step of thermal hydrolysis performed in one or more reactor(s) using a combination of high temperature and high pressure to disintegrate the cellular structure of the organic material in the waste or the sludge and break down high molecular weight organic compounds into smaller molecules.
The step of thermal hydrolysis may be followed by a step of steam explosion performed in one or more pressure relief tank(s) where the content of the tank is disintegrated due to the quick relief of the pressure. The disintegration and splitting up of the biomass makes the following step of fermentation more effective.
The product resulting from pre-treatment steps employing a Thermal Hydrolysis Process (THP) will normally have a high temperature (e.g. above 90° C.) and be characterised by a relatively high dry matter content (e.g. above 25%) and in some instances also a relatively low pH (e.g. below 5). Thus, the handling of this product will normally require highly specialised equipment and in addition it will normally have to be subjected to cooling, neutralisation and/or dilution (e.g. with water) before introduction into a subsequent process based on anaerobic digestion, as this is usually performed at a lower temperature, at a lower dry matter content, and at neutral pH.
WO2007/009463 discloses a method for conversion of cellulosic material, to ethanol and other products. The cellulosic material is subjected to a hydrothermal pre-treatment by at least one soaking operation, a hydrothermal pretreatment in a pressurized reactor, and thereafter a pressing operation, creating a fiber fraction and a liquid fraction. The hydrothermal pretreatment leaves at least 80% of the lignin in the fiber fraction. Due to the need for the handling of material with high dry matter content highly specialized equipment will normally be required in the processes described in WO2007/009463.
WO03/013714 discloses a sluice system by which a product with high dry matter content may be portioned and then conveyed individually through at least one sluice chamber and two pressure locks, thereby allowing for e.g. transfer from a low to a high pressure zone.
Different improvements to the processes for treatment of biomass and organic waste described above have been developed over the years. Thus, one way of achieving these improvements has been by the use of recirculation. In particular recirculation of the steam otherwise used in the process and the use thereof to preheat the biomass, and recirculation of water effluent from the process to reduce the consumption of process water otherwise used in the process, has been described in the prior art.
WO2011/006854 discloses a method and a device for thermal hydrolysis and steam explosion of biomass. The method encompasses steps of preheating the biomass, leading the preheated biomass into at least two reactors where it is heated and pressurised by addition of steam, and finally a step-wise reduction of pressure using two pressure relief tanks. The preheating tank is preheated by return steam from the first and second pressure relief tanks.
WO01/60752 discloses a method, which is a continuous process, involving wet oxidation or steam explosion, for fermentatively converting biomass materials into ethanol. The fermentation wastewater effluent after separation from the produced ethanol, is subsequently subjected to an anaerobic fermentation step generating methane and a wastewater effluent wherein the amount of potentially inhibitory substances is at a sub-inhibitory level, permitting all or part of the effluent water to be recycled into the process to reduce the consumption of process water.
WO2014/039984 discloses a method for treating biomass to obtain monomeric sugars, wherein a pre-treated biomass is subjected to an enzymatic hydrolysis, and at least a portion of the liquefaction material from the enzymatic hydrolysis reactor is recirculated to a location upstream of the addition of the enzymes, as a portion of the coolant for the hot pretreated biomass.
US2009/0098616 discloses a method for treating plant material to release fermentable sugars. The method relates to a two-stage enzymatic hydrolysis process and is preferably preceded by an autohydrolysis step where the material is subjected to high temperature, steam and pressure preferably in the presence of acid. The low-viscosity effluent stream form the first hydrolysis stage is in part recirculated to the first enzymatic hydrolysis stage, some or all directly into the reactor, or it may be mixed with fresh lignocellulosic feedstock prior to entering the reactor. It is further disclosed that the enzymatic process may be performed under vacuum to remove volatile components, such as e.g. enzyme inhibiting compounds like furfural.
Despite the numerous methods of treatment for biomass material, there remains a need for a method where the biomass is pre-treated and subsequently fermented without the need for excess use of chemical additives or specialized equipment for handling dense material with a high dry-matter content, having a high temperature, and a relatively low pH. Additionally, there is a need for a method where dilution with water is minimized at the same time as energy-costs are reduced.